A coil on which an insulation-covered wire rod is wound many times is used for a micro speaker or the like, and a sound is emitted when the coil vibrates by a current. An end part of the wire rod forming the coil is fixed to a terminal of the speaker by caulking or soldering. A coil part itself is fixed by a fusing agent, but the wire rod between the end part and the coil part vibrates due to the vibration of the coil. There is a problem of disconnection of the wire rod if the wire rod has low vibration durability at the time, and thus, high vibration durability is necessary for the wire rod.
In addition, due to the current downsizing of electronic devices, micro speakers are also downsized, and thinning of wire rods is advanced. When a wire rod is thinned, heat is generated due to an increase in electric resistance when a current is made flow and vibration of a coil on which the wire rod is wound. Therefore, heat resistance capable of securing high vibration durability even under a high-temperature environment is also required for the wire rod.
Furthermore, in recent years, a requirement for high-quality sound typified by high-resolution audio is increased. In order to obtain a micro speaker that can also react to such high-frequency vibration, a wire rod having high-cycle vibration durability and heat resistance is required.
Conventionally, a copper alloy wire containing silver has been sometimes used for a wire rod used for a coil. This is because the copper alloy wire containing silver has a property in which strength is improved due to appearance of silver added into copper as crystallized precipitates, and a property in which a decrease in electric conductivity is small even when silver is added into copper while electric conductivity is generally decreased when an additive element is solid dissolved in copper. Before now, as a copper alloy wire which excels in vibration durability, for example, a Cu—Ag alloy wire in which an area ratio of crystallized precipitates having a maximum length of a straight line that cuts the crystallized precipitates of 100 nm or less is 100% is known (PTL 1).